When I have an array of hashes or AR objects, I like to do this to get access to certain fields:
products.map(&:upc_code)
Sometimes I have an array of hashes that are instead keyed by a string, and then I can't use the & sign. So instead I have to do:
people.map{ |p| p['Last Name'] }
I know the ampersand can be used to pass a block, but my question is why does it work the way it does in the first statement above. What is it actually doing?
Actually if no block/proc found in functions like map they call to_proc function for argument. In you example actually goes following conversion:
products.map(&:upc_code) => products.map {|a| :upc_code.to_proc.call(a) }
And actual to_proc method for Symbol looks like (In ruby code):
class Symbol
def to_proc
proc { |o| o.send(self) }
end
end
It return a proc which call a method for ruby object passed as argument. You can override to_proc method for Symbol like below and and to_proc for String to support Hash.
class Symbol
def to_proc
proc { |o|
if o.is_a?(Hash)
o.has_key?(self) ? o[self] : self.to_s.to_proc.call(o)
else
object.send(self)
end
}
end
end
class String
def to_proc
proc { |o| o.has_key?(self) ? o[self] : nil }
end
end
a = [{"x" => 2}, {"x" => 3}]
puts a.map(&:x)
puts a.map(&"x")
Results will be:
=> [2, 3]
=> [2, 3]
Your hash doesnt respond to Last Name, simply. Ie, hash.key doesnt work, its hash[key].
If you use an openstruct, suddenly, it works!
require 'ostruct'
hash = { 'Last Name' => 1 }
[ OpenStruct.new(hash) ].map &:'Last Name'
=> [1]
map(&:key) magically worked for arrays containing hashes that were keyed with symbols, but that isn't the case at all. It's the AR objects whose attributes can be accessed either by symbol or by method. Thanks!
products.map(&:upc_code)is equivalent toproducts.map{ |e| e.upc_code }. A hash doesn't respond to the methods named after keys, thus it doesn't work that way with a hash.